Miscible polymer pair

Polymer alloys are generally named polymer blends within the polymer community. In a recent overview of such blends, Robeson (1994) points out that the primary reason for the surge of academic and industrial interest in polymer blends is directly related to their potential for meeting end-use requirements . He points out that, in general, miscible polymer pairs confer better properties, mechanical ones in particular, than do phase-separated pairs. For instance, the first commercial... 

By rough count, more than 50 miscible systems have been reported in the literature, and from intensive studies of these systems a more optimistic view of the potential for miscibility is beginning to develop. Most of these miscible polymer pairs have chemical structures that are capable of forming strong specific interactions such as donor-acceptor complexes and hydrogen bonds. While they have not all been completely studied, those that have show behavior which can only result from the presence of exothermic or negative heats of mixing. 

Fig. 18. Very slightly miscible polymer pairs often yield an inward shift of their respective glass transition.

Finally, we will consider briefly the formation of multilayer thin films by layer-by-layer deposition of hydrogen-bonded polymer pairs [51,52]. In this way a multilayer structure is obtained from potentially miscible polymer pairs. The stability of these films very much depends on the presence of hydrogen bonds, and pH may be used as an external trigger to erase the layered structure [53,54] and selectively dissolve one of the components [25-27]. This procedure allows for the preparation of microporous films not unlike the nanoporous films obtained by dissolution of the hydrogen-bonded side groups from self-assembled block copolymer-based comb-shaped supramo-lecules [15,17,18]. 

Although investigated in lesser detail earlier, the science and technology of polymer blends had its emergence in the 1970 s. Many of the basic principles existed prior to that time e.g., Flory-Huggins thermodynamic principles as well as contributions by Guggenheim and Prigogine). Commercial blends existed for decades before, however the concept of miscibility, phase behavior, and the basic nature of polymer blends was not well understood or appreciated. An initial review of polymer blends [Bohn, 1968] listed only 12 miscible polymer pairs, some of which were minor variations in copolymer structure. The review also noted that UCST (upper critical solution... 

In the more common polymer pairs with polarity on the elastomer backbone, chemical moieties in the main chain or side groups may attractively interact with each other to enhance miscibility. Paul and Barlow (1978, 1984) indicates that the existence of exothermic interaction (A/f — ve or specific interactions ) is responsible for most of the miscible polymer pairs. In these... 

MSE/MST and MSEE/MST blends were miscible by DSC measurements. Their density vs. composition curves showed positive deviations from the linear additivity rule volume contraction upon mixing two miscible polymer pairs was confirmed. Permeability vs. composition plot also showed large negative deviations from the semilogarithmic mixing rule. The permeability coefficient of the 50/50 MSE/MST blend was the lowest of all the materials tested it is comparable to EVAL-F and much lower than any other commercial barrier polymers. The volume shrinkage in the compatible blends show that the free volume is reduced, thus reducing the permeability. 

Strongly miscible polymer pairs, with % negative over some temperature range, e.g. 

To date, many supposedly miscible polymer pairs [5,6,13,14,42] have been reported in the literature. However, in some cases [13,14], the breadth of the glass transition region, ATg, taken as the difference between the onset and completion temperatures, is quite broad. For some blend systems, ATg values approach 100°C [13,14]. The transition region may also be... 

Sp6CifiC Interactions. In order to enhance miscibility between two polymers, the enthalpic term has to be made negative by introducing specific interactions between the two components. A wide range of miscible polymer pairs... 

For a miscible polymer pair, the blend permeability Ph was empirically foimd to approximately obey the semilogarithmic additivity rule (302) ... 

There are many other examples of miscible polymer blends arising from specific submolecular interactions of the components. The miscible polymer pair of polystyrene and poly(phenylene oxide), for example, was recently studied by the 2D FT-IR technique in an effort to understand the molecular origin of the miscibility of this system [55]. Specific interactions between polymers and various small molecules, such as additives and plasticizers, were also studied by 2D IR. The importance of localized submolecular level interactions among functional groups of mixture components was demonstrated. 

Krause [5] reviewed the miscible polymer pairs reported in the literature. She found that there were 282 chemically dissimilar polymer pairs that appeared to be miscible in the amorphous state at room temperature. She found that 15% of these were miscible on account of specific interactions such as hydrogen bonding 15% of the miscible pairs were due to clever choice of copolymer composition. The compositional window over which copolymers are miscible with each other can be calculated from the Flory-Huggins theory. 

Most polymer pairs are immiscible that is, mixtures of the two remain phase separated. This is caused by the (usually) positive heat of mixing, and very small entropy of mixing, as developed in Section 4.3. A list of selected immiscible polymer pairs is shown in Table 13.1 (3). A corresponding list of miscible polymer pairs is shown in Table 13.2 (3). 

BHA Bhattacharyya, C., Maiti, N., Mandal, B.M., and Bhattachaiyya, S.N., Sensitivity of ternary phase diagrams of miscible polymer pairs in common solvents to traces of water Solvent-poly(ethyl aciylate)-poly(vinyl propionate) and solvent-poly(methyl aciylate)-poly(vinyl acetate) systems. Macromolecules, 22,487, 1989. 

This effects depend on the alloy composition, choice of components, and their ratio. By an appropriate choice of components and knowing the polymer-solid interaction parameter, %s, the pairs may be selected for which introduction of particulate fillers will lead to increased compatibilization. There exists some analogy with improvement of miscibility of two polymers by introducing a third pol3mier miscible with each of the polymer components of the blend. We should like to note that for miscible polymer pairs the filler decreases the compatibility. This effect is probably a result of interaction as5mime-try. 

Experimentally, there are many miscible polymer pairs in which at least one of the components is a random copolymer but specific interactions are not present [40]. This phenomenon is attributed to the so-called intramolecular repulsion effect. Within the familiar Flory-Huggins description, and in the case of a random copolymer A Bi blended with a homopolymer C, three interaction parameters, Xab,Xac and Xbc are required to describe the enthalpy of mixing. Using a mean field theory, the mixture can be described in terms of one parameter XeS given by ... 

Whereas examples in Fig. 19.6A,B pertain to immiscible polymer melts. Fig. 19.6C pertains to a miscible polymer pair [31]. Submicron layers were formed in a blend of 30% by volume EVOH and polyamide (PA). To improve phase contrast in the scanning electron image, samples were etched in a solvent so that some or portions of EVOH layers were removed. Thicknesses of layers were determined by separate transmission electron examinations to be less than 0.5 pm. The potential amount of layer refinement obtained with a smart blender is demonstrated for a blend consisting of 15% by volume LDPE and high density polyethylene (HDPE) [5]. Because interfacial tension was very low due to... 

In case of immiscible polymer pairs the ternary mixture is more stable thermodynamically (their adsorption on the surface of solid particle is more preferred than that of a miscible polymer pair). This is the basis of the nanocompatibilization (i.e. compatibilization with the aid of nanoparticles). Performance fire retardant system of polypropylene (PP) and phosphorylated epoxy resin (PEP) was reported to improve when montmorillonite nanoclay was introduced. Without nanoclay the distribution of epoxy in PP matrix was inhomogenous, while homogeneous nanodispersion could be achieved after attachment of PEP to clay nanoparticles as interlayer. (8). 

Abstract A review of definitions and the overall rationale for the production of high temperature polymer blends is provided.The discussion is divided essentially into two parts miscible and immiscible blends. It is pointed out that one concern with miscible polymer pairs is that of processing in the miscible state. This phenomenon is dependent on the position of the phase separation temperature relative to the glass transition temperature of the polymer blend. In the case of immiscible blends, the issue of adhesion of the polymers is discussed. Finally, the need for better theoretical models for the prediction of miscibiUty in polymer blends is highhghted and discussed. 

In fact, this topic has evolved into a central area of polymer research during the last 40 years. One of the first ideas, that as a rule polymer blends are immiscible, needs to be reevaluated due to the increasing number of miscible or partially miscible polymer pairs reported in the literature (see, for example, Paul and Newman, 919) Despite this high level of activity, much of the work remains based on art and intuition rather than on science. Most of the work performed on high temperature polymer blends has involved the definition of miscible polymer pairs and their phase separation characteristics. Not much work has been done to predict miscible pairs. In fact, this is an open area of research in the entire area of polymer blends. 

By varying the composition of a blend, the engineer hopes to obtain a gradation in properties that might be tailored for specific applications. This is true for miscible polymer pairs such as polyphenylene oxide and polystyrene that appear and behave as single-component polymers. 

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